1. Field of the Invention
The invention relates to a method of spectrum analysis, more particularly to a method for optimum spectrum analysis.
2. Description of the Related Art
Conventional harmonic measuring equipments, such as a spectrum analyzer, a harmonic analyzer, a distortion analyzer and a digital harmonics measuring device, utilize Fast Fourier transformation for spectrum analysis. However, when a harmonic frequency of a sample signal is not a multiple of frequency resolution, there are a grating effect due to frequency distortion and a leakage effect due to amplitude distortion generated in a spectrum transformed from the sample signal by Fast Fourier transformation.
The following are two methods proposed in the art to reduce the grating effect and leakage effect. A window method of multiplying a sample signal by a window function is used to overcome discontinuation at truncation points in a spectrum so as to eliminate side lobe components in the spectrum. Although the window method can reduce the leakage effect, characteristics of spectrum have been changed. A zero padding method can be used to solve the grating effect but cannot improve the leakage effect due to the decreased amplitude.
Moreover, in order to maintain the characteristics of the spectrum, a method has been proposed heretofore to select an updated graduation interval that is a common factor of harmonic frequencies. However, there is a relatively great difference between the common factor and an original graduation interval such that the method cannot be used in an actual practice. Thus, the updated graduation interval is not an optimum graduation interval.